The pigeon command generates parsers based on a parsing expression grammar (PEG). Its grammar and syntax is inspired by the PEG.js project, while the implementation is loosely based on the parsing expression grammar for C# 3.0 article. It parses Unicode text encoded in UTF-8.

See the godoc page for detailed usage. Also have a look at the Pigeon Wiki for additional information about Pigeon and PEG in general.

• Performance tweak, 5-10x faster than original version(with some trade off).

  • parser.state is removed, because it's very slow.
  • parseSeqExpr only collect not nil values now. Mainly for performance improvement. For example: e <- Expr __ Plus __ Expr returns [expr, '+', expr], original version return [expr, nil, '+', nil, expr].
  • Memoized can be worked with Optimize and enable by default.
  • Generated parser do less memory allocated.
  • Generated parser uses fewer lines of code.

• Multiple peg files supported.

  1. pigeon -o script1.peg.go script1.peg to generate a normal parser.
  2. Run pigeon -grammar-only -grammar-name=g2 -run-func-prefix="_s2_" -o script2.peg.go script2.peg to generate grammar only code in same package.
  3. Use it by newParser("filename", "expr").parse(g2)

• actionExpr is different

  • Only needs to return one value. Moreover, this is not required. If the return statement is not written, it will automatically return nil. Examples:
    • expr <- [0-9]+ { fmt.Println(expr) } is ok in this fork.
    • expr <- "true" { return 1 } if you want return something.
  • If you want to add an error by manual, do this:
    • expr <- "if" { p.addErr(errors.New("keyword is not allowed")) }, equals to expr <- "if" { return nil, errors.New("keyword is not allowed") } of original pigeon.

• andCodeExpr and notCodeExpr:

  • Like actionExpr, return a bool instead of return bool and error
  • expr <- &{ return c.data.AllowNumber } [0-9]+

• String capture:

  • expr <- val:<anotherExpr> { fmt.Println(val.(string)) }
  • expr <- val:<(A '=' B)> { fmt.Println(val.(string)) }

• Logical and / or match:

  • expr <- &&testExpr testExpr // if testExpr return ok but matched nothing (e.g. testExpr <- 'A'*), &&testEpr returns false.

• Skip "actionExpr" while looking ahead issue, branch feat/skip-code-expr-while-looking-ahead

  • See detail in the issue.
  • *{} / &{} / !{} won't skip.

• Remove ParseFile ParseReader, rename Parse and all options to lowercase issue, branch feat/rename-exported-api

  • ParseReader converts io.Reader to bytes, then invoke parse, it don't make sense.
  • Function Parse and all options(MaxExpressions,Entrypoint,Statistics,Debug,Memoize,AllowInvalidUTF8,Recover,GlobalStore,InitState) expose to module user. I think expose them is not a good idea.

• ActionExpr refactored issue, branch refactor/actionExpr

  • Unlimited ActionExpr(CodeExpr): grammar like expr <- firstPart:[0-9]+ { fmt.Println(firstPart) } secondPart:[a-z]+ { fmt.Println(firstPart, secondPart) } is allowed for this fork.
  • You can access parser in ActionExpr: expr <- { fmt.Println(p) }
  • stateCodeExpr(#{}) was removed.

• Provide a struct(ParserCustomData) to embed, to replace the globalStore

  • Must define a struct ParserCustomData in your module.
  • Access data by c.data, for example: expr <- { fmt.Println(c.data.MyOption) }
  • globalState is removed.

• position of generated code is removed

  • It produced a lot of different for version control.
  • You can keep it by set SetRulePos to true and rebuild.

• Added -optimize-ref-expr-by-index option

  • An option to tweak RefExpr the most usually used expr in parser.
  • About ~10% faster with this option.

• Removed -support-left-recursion option

  • It's not used much, so I removed it to make maintenance easier

• Removed -optimize-grammar option

  • There are bugs present and the effects are not significant.

• Removed -optimize-basic-latin option

  • Because there is no evidence to suggest that this is an optimization

• charClassMatcher / anyMatcher / litMatcher not return byte anymore, because of performance.

  • Use string capture or c.text instead.

• v1.0.0 is the tagged release of the original implementation.
• Work has started on v2.0.0 with some planned breaking changes.

Github user @mna created the package in April 2015, and @breml is the package's maintainer as of May 2017.

Starting of June 2023, the backwards compatibility support for pigeon is changed to follow the official Go Security Policy.

Over time, the Go ecosystem is evolving.

On one hand side, packages like golang.org/x/tools, which are critical dependencies of pigeon, do follow the official Security Policy and with pigeon not following the same guidelines, it was no longer possible to include recent versions of these dependencies and with this it was no longer possible to include critical bugfixes. On the other hand there are changes to what is considered good practice by the greater community (e.g. change from interface{} to any). For users following (or even enforcing) these good practices, the code generated by pigeon does no longer meet the bar of expectations. Last but not least, following the Go Security Policy over the last years has been a smooth experience and therefore updating Go on a regular bases feels like duty that is reasonable to be put on users of pigeon.

This observations lead to the decision to follow the same Security Policy as Go.

Provided you have Go correctly installed with the $GOPATH and $GOBIN environment variables set, run:

$ go get -u github.com/mna/pigeon

This will install or update the package, and the pigeon command will be installed in your $GOBIN directory. Neither this package nor the parsers generated by this command require any third-party dependency, unless such a dependency is used in the code blocks of the grammar.

$ pigeon [options] [PEG_GRAMMAR_FILE]

By default, the input grammar is read from stdin and the generated code is printed to stdout. You may save it in a file using the -o flag.

Given the following grammar:

{
// part of the initializer code block omitted for brevity
type ParserCustomData struct {
}

var ops = map[string]func(int, int) int {
    "+": func(l, r int) int {
        return l + r
    },
    "-": func(l, r int) int {
        return l - r
    },
    "*": func(l, r int) int {
        return l * r
    },
    "/": func(l, r int) int {
        return l / r
    },
}

func toAnySlice(v any) []any {
    if v == nil {
        return nil
    }
    return v.([]any)
}

func eval(first, rest any) int {
    l := first.(int)
    restSl := toAnySlice(rest)
    for _, v := range restSl {
        restExpr := toAnySlice(v)
        r := restExpr[3].(int)
        op := restExpr[1].(string)
        l = ops[op](l, r)
    }
    return l
}
}


Input <- expr:Expr EOF {
    return expr
}

Expr <- _ first:Term rest:( _ AddOp _ Term )* _ {
    return eval(first, rest)
}

Term <- first:Factor rest:( _ MulOp _ Factor )* {
    return eval(first, rest)
}

Factor <- '(' expr:Expr ')' {
    return expr
} / integer:Integer {
    return integer
}

AddOp <- ( '+' / '-' ) {
    return string(c.text)
}

MulOp <- ( '*' / '/' ) {
    return string(c.text)
}

Integer <- '-'? [0-9]+ {
    v, err := strconv.Atoi(string(c.text))
    if err != nil {
        p.addErr(err)
    }
    return v
}

_ "whitespace" <- [ \n\t\r]*

EOF <- !.

The generated parser can parse simple arithmetic operations, e.g.:

18 + 3 - 27 * (-18 / -3)

=> -141

More examples can be found in the examples/ subdirectory.

See the package documentation for detailed usage.

See the CONTRIBUTING.md file.

The BSD 3-Clause license. See the LICENSE file.

• performance: Create another version of parseOneOrMoreExpr/parseZeroOrMoreExpr which not collect results. Choose expr decide by is labeled, A bit faster.
• performance: Remove pushV and popV, a bit faster.
• performance: In parseCharClassMatcher, variable start can be removed in most case. Lot of of small memory pieces allocated.
• performance: Remove Wrap function if they are not needed.
• performance: Too many any, can we remove parseExpr?